Note: Descriptions are shown in the official language in which they were submitted.
This in~ention relates to a method of graining the surface of
aluminum sheets which are useful in the production o lithographic printing
plates.
In the manufacture of lithographic printing plates, it has been
found most desirable to employ aluminum or aluminum alloy sheets as the base
support there~or. In addition, it has been found that most satisfactory
aluminum lithographic plates are obtained when the surface of the aluminum or
aluminum alloy base support sheet is treated to impart thereto a grained or
roughened character. Heretofore, a number of methods have been employed to
impart a grained surface to the aluminum base support sheet, including both
mechanical and electro-chemical processes.
The mechanical graining method of treating aluminum sheets, for
example, by wire brushing, results in a grained surface which is relatively
rough and uneven, and in many lithographic printing applications does not
give satisfactory results. It is also known that the electrolytic graining
of aluminum can provide a fine and uniform grain to the surface of the aluminum.Various methods of electrolytically graining aluminum sheets have been
employed, for example, as taught by U.S. Patents 3,072,546, 3,073,765 and
` 3,980,53g, and French Patent 2,110,257. In some of the prior art processes
employed it has been found that unless the process is carefully controlled,
the resultant grained surface obtained can be pitted, coarse and irregular,
characteristics which are not desirable in lithographic printing plates.
We have now found a method for promptly and efficiently eletrolytically
graining the surface of aluminum sheets in such a manner as to yield
aluminum sheets which have a very fine and uniform grain surface which is
most desirable for use in the production of lithographic printing plates.
More particularly, the process of this invention comprises electrolytically
graining aluminum in an aqueous electrolyte solution containing hydrochloric
acid and tartaric acid with an electric current yielding a current density
in excess of 40 amperes per square foot, at a temperature in excess of 45 C,
the concentrations of hydrochloric acid and tartaric acid being suffi-
cient that a fine, uniform grain that is substantially free from pits
is formed on the surface of the aluminum thus treated.
m e aluminum which is contemplated to be employed is that alumi-
num or aluminum alloy which is designed and intended for employment in
the production of lithographic printing plates. Thus, the aluminum
to be employed herein is in the form of such aluminum sheets and webs
as are specifically designed for use in the manufacture of lithograp-
hic printing plates, and includes such aluminum as is produced and
sold by the Aluminum Company of America as lithographic grade Alloy
No. 3003, or Alloy No. 1100, as generally known and understood in the
industry.
The aluminum may then be electrolytically treated in accordance
with the process of this invention. The electrolytic solution em-
ployed in the practice of this invention is an aqueous electrolytic
solution which requires the presence, in combination, of a ~mall but
effective amount of hydrochloric acid and a small but effective amount
of tartaric acid, as the active electrolytes. More specifically, it
has been found that most successful results are obtained when the
aqueous electrolytic solution contains concentrated hydrochloric acid
in combination with tartaric acid. It has also been found that satis-
factory results are obtained when the concentrated hydrochloric acid
(defined as containing at least 32% HCI by weight) is present in the
electrolytic solution in a concentration of from at least 0.75% to
about 3.5% by weight, and most preferably, in a concentration of
from 1.5% to 2.5% by weight. The tartaric acid electrolyte should
be present in the aqueous electrolytic solution in a concentration
of at least 0.2% to about 1.0% by weight and preferably, in a concen-
tration of from 0.3% to 0.75% by weight.
The electrolytic current which is employable in the practice of
this invention is that which will provide a current density in ex-
cess of 40 amperes per square foot. Most satisfactory results can be
obtained
when the current applied in the electrolytic graining process of this
invention provides a current density of from 200 to 500 amperes per square
foot of aluminum surface being treated, and most preferably, a current density
of from 250 to 350 amperes per square foot.
It has also been found in the practice of this invention that the
temperature at which the process is operated is critical in achieving the
desired results. The temperature at which the electrolytic graining process
is conducted must be maintained at a high enough level to assure that a fine,
uniform grain is obtained It has been determined that satisfactory results
are obtained when the temperature of operation is maintained above ~5 degrees
C. and preferably between 45 degrees C. and 75 degrees C. If the temperature
at which the electrolytic graining is conducted is too low, for example,
below 40-45 degrees C. the grain obtained is undesirably rough and not
usually employable in the production of lithographic printing plates.
The electrolytic graining process of this invention may be carried
out in a batch, semi-continuous or continuous manner, employing the aluminum
to be treated hereunder in the form of either sheets, foils or in continuous
webs, as may be desired by the skilled worker. While the amount of time
required for the completion of the process of this invention may vary,
according to the conditions of operation under which it is practiced by the
skilled worker, it has been found that satisfactory results can be obtained
in a time period as little as thirty seconds. Most satisfactory results
have been obtained when the process is practiced for from 60 to 90 seconds,
although other periods of operation also provide satisfactory results.
The invention is further illustrated by the following examples.
EXAMPL~ 1
A piece of aluminum foil 0.4 mm thick and measuring 4 inches
square was immersed in a 5% w/w solution of NaOH for 30 seconds at room
temperature to clean the surface thereof. The aluminum alloy was purchased
as lithographic grade aluminum Allay No. 3Q03 from the Aluminum Company of
8~
America. The thus treated aluminum was then washed and immersed in an
electrolytic bath containing 1.75% by weight of concentrated hydrochloric
acid and 0.5% by weight of tartaric acid in deionized water. An alternating
current at 15 volts and a current density of 300 amperes per square foot
was passed ~rom the foil through the electrolyte to a counter electrode for
a period of one minute. The temperature of the electrolytic bath was main-
tained at about 55 degrees C. during the process. Only one side of the
aluminum foil sample was grained, the back thereof being effectively masked.
The oil was then washed with wa~er.
EXAMP~E 2
The procedure of Example 1 was followed except that the tartaric
acid electrolyte was omitted from the electrolytic solution. The resultant
grained foil was obtained and the surface roughness of the two foil samples
were compared by Perth-O-Meter tTrademark of Perthen Co.). Higher reading
indicating rougher surface,
Grained Surface Foil Roughness Value
Example 1 6
Example 2 7.5
The foregoing results demonstrate that the process of the instant
invention provides a smoother grain.
The invention may be variously otherwise embodied within the scope
of the appended claims.
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